Add radicle-git

[lorenzleutgeb]

Radicle is an open source, peer-to-peer code collaboration stack built on Git.

Repositories are addressed by hashing their initial "Repository Identity Document".

In order to allow addressing repositories by CIDs, reserve a new code.

For example, we currently use rad:z3gqcJUoA1n9HaHKufZs5FCSGazv5 to address the repository which you can view on the web at https://radicle.network/nodes/seed.radicle.dev/rad%3Az3gqcJUoA1n9HaHKufZs5FCSGazv5.
The identifier here is what we call a "Repository Identifier" and it is a plain base56btc encoding of a special Git object in that repository.

With the imminent introduction of SHA-256 as the default object format/hash function in Git 3, we plan to introduce more flexibility into our repository identifier scheme. CIDs with this multicodec code look very promising.

[2color]

Since the hash that we intend to encode in the CID is of a Git object, I wonder if it makes sense for us to use raw-git:

multicodec/table.csv

Line 48 in 45c88b8

git-raw, ipld, 0x78, permanent, Raw Git object

[lorenzleutgeb]

I am not sure in how far semantics matter in this case. Sure, both things seem to capture SHA-1 and SHA-256 hashes, with the intention of identifying a Git object. But what about the intent/semantics of wanting to address a repository? This differs from the intent to address an object. Maybe someone can weigh in on how to balance such semantic differences.

For example, I have relatively good knowledge about what it would mean to address a Radicle repository (this currently happens, just using different syntax). But TBH I have little clue about how the git-raw codec is to be interpreted.

[vmx]

This PR reminded me of the discussion at #203 about adding a custom code, although things are really Git.

In regards to what IPLD codecs are, see my comment at #204 (comment).